The Six Moons Most Likely to Host Life in Our Solar System Vast quantities of liquid water may exist

Vast quantities of liquid water may exist on moons of Jupiter, Saturn and Neptune, making life possible there, too

By Rebecca Boyle, Juan Velasco on May 1, 2023
Scientific American May 2023 Issue

In 2005 the Cassini spacecraft visiting Saturn flew through something engineers didn’t expect—a fine water mist, spraying into space at 1,290 kilometers per hour through cracks in the surface of Saturn’s tiny, ice-covered moon Enceladus. Cassini wasn’t designed to sample the water, but the discovery inspired scientists to develop new missions to the outer solar system’s icy moons. At least six of those worlds—two orbiting Saturn, three orbiting Jupiter and one by Neptune—might host watery oceans, sandwiched between a warm planetary core below and ice crust above.

On Earth, water is required for life “as we know it.” Other than the dunes of Mars, where we have searched for half a century, astrobiologists now consider the icy moons of the outer planets some of the best places to look for life in our solar system.

The European Space Agency’s Jupiter Icy Moons Explorer, nicknamed JUICE, was scheduled to launch in April toward the gas giant and its moons Europa, Callisto and Ganymede. JUICE and NASA's Europa Clipper mission to Jupiter and Europa, set to launch in 2024, will change our understanding of the outer solar system. The icy moons may rewrite our cosmic perspective, just as they did when astronomers discovered them in the 17th century.

“The outer solar system is probably replete with moons that could have liquid water oceans on them, and a subset could have geothermal and water-rock interactions on the bottom,” says Chris German, an oceanographer at the Woods Hole Oceanographic Institution, who is co-leading a NASA-funded initiative called Network for Ocean Worlds (NOW). Why do those characteristics matter? “Everywhere that has those on our planet gets colonized by microbial life,” German says.

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